Controlled casting of a shrinkable material

ABSTRACT

A method and a device are disclosed wherein a shrinkable polymer material is formed in situ in a mold without defects and with no internal stresses. A monomer or polymer solution is injected into the mold and solidified sequentially through the mold by exposure to an agent such as ultraviolet radiation, with simultaneous addition of monomer or polymer solution into the area of the mold not yet exposed to the solidifying agent. By controlling the rate at which the solidifying agent is moved across the mold and the monomer or polymer solution is injected into the mold, the resulting product completely fills the mold and is stressfree.

TECHNICAL FIELD

This invention relates to controlled solidification of a pre-polymermixture in a precisely dimensioned confined space. In particular, itrelates to the manufacture in a confined space of precisely dimensionedstress free articles made of polymers either by in situ polymerizationor by devolatilization or coagulation of a dissolved polymer mixture

BACKGROUND OF THE INVENTION

The manufacture of molded plastic parts made of polymers can be broadlystated as taking place in two distinct steps. The monomeric materialused to make the polymer is usually polymerized separately prior to theshaping stage of the plastic part. There are, of course, exceptions;however generally speaking the polymer, once made, is ground intorelatively fine particles or pelletized and then heated to liquefy thepolymer or alternatively a solvent is added to dissolve the polymer.Subsequently, the liquefied polymer is either injected into a mold,extruded, spun or in some instances blow molding takes place. There aresituations where direct casting is performed; however, in casting, andfor that matter in injection molding, there exists an unacceptabledegree of shrinkage and flow-induced molecular orientation for the mostprecisely dimensioned parts. When a solvent is used to liquefy a polymerfor later solvent devolatilization in an attempt to obtain stress freegeometric shapes, concomitant shrinkage caused by solvent evaporationresults in cracks and unacceptable stresses in the hardened polymer.

Similarly, if in situ polymerization is attempted, the pre-polymerreaction mixture or raw material that will make up the polymer mayshrink upon polymerization up to 20 percent. Thus, neither in situpolymerization or molding of an already formed polymer has provedsuccessful in manufacture of precision stress free plastic parts. Tocompound the problem, when the molded part exceeds certain thicknesses,cavitation due to shrinkage will frequently leave unacceptable bubblesin the part. These bubbles are indicative of internal stresses.

Three exemplary fields exist where polymers are appropriate for use;however, the limitations set forth above, that is shrinkage of polymersat mold time or at polymerization time, limits usage of polymers forprecision articles. Similarly shrinkage of a dissolved polymer upondevolatilization also limits use of parts formed in this method.

When the article is to be used in an optical application, e.g., a largelens, a second requirement, in addition to precise dimensions, ispresent. That is, there can exist no internal stresses in the article asinternal stresses will result in birefringence. Such is the case ininjection molding of large plastic lenses and injection molding ofoptical or magnetic data storage discs currently used in compact discrecorders and personal computers and anticipated as being used asstorage media for data in other computer systems. Both these productsare now injection molded; however, both with the above restrictions. Inthe case of the lenses, it is common to utilize a glass blank of a sizesomewhat smaller, but generally conforming to the lens curvature desiredin the final product. In this instance the monomer is polymerized aboutthe glass blank in a relatively thin film. However, blanket exposure ofthe entire lens leaves bubbles and internal stresses that would occuranywhere but more frequently in narrow spaces. These defects appearoften in the center or the thickest part of the lenses if the lenseswere made entirely of the plastic material. The present inventionaddresses such problems in both types of construction.

In the case of the disc, injection molding generally results in someflow-induced and thermoplastic internal stresses. Such stresses causebirefringence and thus the storage capacity and data detectionreliability of the disc are limited.

In the third example, a liquefied polymer sheet, when cast onto a flator curved support will warp or crack if allowed to devolatilize orcoagulate over the entire surface simultaneously. Such warping orcracking is caused by shrinkage due to solvent loss.

This invention discloses a method for in situ formation of preciselydimensioned precision parts made of polymers either by in situpolymerization or differential devolatilization or coagulation.

It is an object of this invention to provide a method that permits insitu molding of parts formed of polymers without internal stresses.

It is also an object of this invention to provide a method that permitsdifferential polymerization in a precision mold thereby eliminatingvoids caused by shrinkage.

It is a further object of this invention to provide a method of castinga polymer sheet by devolatilization or coagulation that avoids surfacecracking of the film.

It is also an object of this invention to provide a method that permitsin situ polymerization of relatively thick and large precision partswithout internal bubbles.

It is still another object of this invention to provide a method thatpermits casting of precision polymer parts with widely varyingthicknesses.

It is a object of this invention to provide an method for differentialpolymerization of variable thickness precision parts.

SUMMARY OF THE INVENTION

This invention is a method for forming an article of precise dimensionsby in situ solidification in a precisely dimensioned mold of a liquefiedmaterial that upon exposure to a solidifier shrinks.

The method comprises the steps of providing a mold body having a firstclosed end and a second open end and defining an internal cavitycorresponding to the precise dimensions of the finished article. Themold body is so formed that it may be differentially exposed to asolidifier starting at the first closed end and moving in a controlledmanner to the second open end. The method also includes the step ofproviding a source of the solidifier for imposition upon the liquefiedmaterial within the mold body. It also includes the step of providing aconstant source of liquefied material at the open end of the mold body.Finally, it includes the step of differentially exposing the liquefiedmaterial to the solidifier starting at the closed end and proceeding tothe open end while continuously supplying liquefied material to the openend. The invention also includes a device for forming an article of amaterial that upon exposure to a solidifier shrinks The device comprisesa mold body defining an internal cavity having a portion of the moldbody so formed that the internal cavity is differentially exposeable toa solidifier, the mold body having a first end and a second end with theinternal cavity conforming to the desired outer dimensions of thearticle, the mold body further includes a gate at the second end of thecavity, the gate communicating with the cavity. The device also includesa source for the solidifier and means for focusing the solidifier upon aselected area of the material such that it is imposed differentiallyupon the material in the internal cavity. Means are also provided formoving the solidifier imposed upon the material in the internal cavityrelative the internal cavity from the first end to the second end at acontrolled velocity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of a device capable of differentialexposure of a solidifier to a mold body.

FIG. 2 is a perspective view of the mold shown in FIG. 1 with thesolidifier removed.

FIG. 3 is an alternate embodiment of the mold body shown in FIG. 1.

FIG. 4 is an illustration of a portion of a mold body whereinpolymerization takes place across the entire molded body at one time.

FIG. 5 is a mold body for in situ devolatilization or coagulation of apolymer in a differential manner.

FIG. 6 is a cross-sectional view of the mold body shown in FIG. 5 atcut-line 6--6.

FIG. 7 is a device for molding curved bodies through devolatilization.

FIG. 8 is a cross-sectional view of the embodiment shown in FIG. 7wherein the cut-line in FIG. 7 is at 8--8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1 a mold body 10 is shown in cross-section. The moldbody as shown in FIG. 1 is designed specifically for a lens shapedprecision article such as a projection lens for a television receiver.The device 10 shown in FIG. 1 is formed of at least two parts 12 and 14,brought together to form a cavity 16. Cavity 16 is formed having theshape of the precision body that is desired to be molded. As is usualwith a mold, a gate 18 provides access to the mold body 10 when thefirst and second part are engaged. Communicating with gate 18 is areservoir 20 which is utilized to feed raw material to cavity 16 throughgate 18. Reservoir 20 is represented in FIG. 1 as a hopper-like device.A vent 21 (see FIG. 2) may also be included to facilitate the filling ofcavity 16. It should be understood that other means for providing rawmaterial to cavity 16 through gate 18 may be advantageously used. Forexample, it may be appropriate to provide raw material to cavity 16under pressure.

Mold body 10, as can be seen in FIG. 1, necessarily has one part, in thecase illustrated, part 14, that is transparent to a source of energy. Asource of energy 22 is movable relative to mold body 10 and includes afocusing means such as gate 24. The source of energy 22 may be drawnacross the second part 14 by means of a two-way motor 26. Source ofenergy 22 is selected according to the material to be molded. Forexample, if the monomers (sometimes referred to as the reaction mixtureor polymer precursor) provided to the mold cavity 16 from reservoir 20are to be polymerized by heat, then source of energy 22 is appropriatelya heat source which is focused through an opening 23 in focusing gate24. Opening 23 is preferably designed to focus a plane of energy onsecond part 14. The plane of energy is substantially normal to themovement of focusing gate 24. Alternatively if the monomers utilized incavity 16 are polymerized by an ultra violet source or other lightsource, then source of energy 22 may be a light of the proper wavelength. Again, second part 14 is of necessity transparent to the wavelength of light utilized in source of energy 22 in the eventpolymerization takes place under the imposition of a light source. Inthe event that polymerization takes place as a result of the impositionof heat, second part 14 is appropriately thin and made of material thathas little or no insulative qualities. It may also include passages 34for cooling. These passages, as will be seen, may be selectively used sothat a time-dependent temperature gradient will be maintained. Similarpassages may be symmetrically located in part 12 (not shown).

Important to the invention is the fact that solidification of theliquefied material in cavity 16, be it monomers that are to bepolymerized or a polymer which has been liquefied by a solvent is thedifferential exposure of the material in cavity 16 to the solidifier. Inthe case of the polymerization, the solidifier is the source of energy22, while in the case of devolatilization or coagulation of a dissolvedliquefied polymer mixture, best illustrated in FIG. 5, theambient-controlled atmosphere or a non-solvent extraction bath is thesolidifier with exposure of the material in cavity 16' to the atmospheredone by removal of the second part 14' from the first part 12' of themold body 10'. More will be said about the alternate embodiment in theensuing discussion.

Returning now to FIG. 1, movement of focusing gate 24 relative to moldbody 10 is controlled so that source of energy 22 scans across the moldbody 10 starting at the closed end 26 of cavity 16 and moving towardgate 18. Should cavity 16 be exposed to a source of energy 22 across theentire surface, which is the normal process in polymerization, therewould be severe shrinkage in the mold body and a strong likelihood thatbubbles would form in the formed polymer especially in highly stressedportions. Reference is made to FIG. 4 wherein an example is shown ofwhat would occur with simultaneous polymerization within the mold body.It is to be understood that second part 12 has been removed in FIG. 4and the molded form 28 is shown with a void 30 at its upper end adjacentto gate 18. Additionally, bubbles 32 formed by cavitation duringpolymerization also may very likely occur in the molded body 28. Thevoids 30 and bubbles 32 are attributable to the chemical linkageformation during polymerization. In order to avoid these problems, ithas been found that providing a continuous source of monomer or reactionmixture to be polymerized at reservoir 20 and in turn to gate 18 willavoid the formation of unacceptable bubbles and voids. Since monomersgenerally have a relatively low viscosity, they will flow easily throughgate 18 into cavity 16 to fill the volume lost to shrinkage of thereacting mixture.

In the present invention the reaction mixture which is contained inreservoir 20 is constantly resupplied to cavity 16 through gate 18 thusas polymerization occurs at the lower end or closed end 26 of mold 10the shrinkage that occurs and would eventually appear as a void 30 asshown in FIG. 4 is immediately replenished by the reaction mixture ormixture of polymers contained in reservoir 20. It is of courseunderstood that the reaction mixture is highly mobile and flows readilyto fill the volume lost due to shrinkage of the part of the mixture thathas already undergone reaction. The instantaneous replacement of thespace formed by shrinkage by unreacted material ensures a final piecethat is defect free and distortionless. The movement of the energysource 22 relative to the mold body 10 must, of necessity, start withopening 23 in focusing gate 24 moving from closed end 26 to gate 18 in amanner such that polymerization takes place at a steady rate from theclosed end to the gate end.

In the event the source of energy 22 is by the nature of the monomer aheat source, movement of the focusing gate across the mold body 10 mustbe at a rate that does not permit heat transmission through second part14 at a rate faster than polymerization is taking place. That is, as theheat source of energy 22 moves upwardly, second part 14 will absorb heatand conduct that heat inwardly to cavity 16 where polymerization takesplace. The portion of second part 14 above opening 23 must be kept coolwhich may be accomplished by circulating a cooling fluid throughpassages 34 so that the upper portion of part 14 remains cool inrelation to the lower portion of part 14 thereby providing differentialheating of mold cavity 16. Similar cooling passages may be appropriatein the upper portion of part 12 (not shown).

Referring specifically to FIGS. 1 and 2, the exemplary molded body 36shown therein is a convex-convex lens.

Referring to the alternate embodiment in FIG. 3, wherein the energysource 122 is fixed and the mold body 110 is movable relative to thesource of energy, a different type of exemplary cavity 116 isillustrated. The cavity 116 is a flat, disc-like cavity which isappropriate for optical or magnetic discs utilized currently inrecording of music and the like and for storage of data in computerizedsystems. Currently, these optical or magnetic discs are injection moldedwhich imparts residual stresses yielding a product that may warp withtime. Birefringence, which occurs in injection molded optical datarecording substrate is also highly undesirable. Here again, the reactionmixture is provided to cavity 116 through gate 118 as a continuoussource avoiding the stresses inherent in injection molding. (Ininjection molding processes, the material provided to the mold cavity isgenerally thermoplastic with a high distortion temperature that hasalready been polymerized then pelletized and melted. The injectionmolding takes place under extremely high pressure, followed byrelatively fast cooling with almost an assurance of inherentlyunacceptable residual stress for the manufacture of optical or magneticdiscs.)

In FIG. 3, the mold body 110 with its first part 112 and second part 114transparent to source of energy 122, is moved relative to source ofenergy 122 with polymerization occurring first at the closed end 126 andmoving across cavity 116 to gate 118 by the relative movement of thesource of energy and the mold body. Here again, second part 114 istransparent to the source of energy be it heat or a particular type oflight. In situ polymerization in this instance may be more adaptable toheat triggered reaction as the control of the incidence of heat upon thecavity 116 where the molded body 136 is of a uniform thickness is morereadily controlled. Here again the unreacted mixture contained inreservoir 120 communicates with cavity 116 through gate 118 to fill thevolume lost due to reaction shrinkage thereby compensating for anyshrinkage in the disc. The final product is a perfectly shaped disc withall portions in a stress free state and free of any internal voids. Itlacks birefringence and residual distortion and is dimensionally exactto the degree provided in cavity 116.

This method of preparation of precision molded polymeric parts appliesto all monomers, monomer mixtures, and monomer/cross linker mixtures. Asresult, this method permits the broadest selection of the reactionchemistry to achieve precision parts with the required mechanical,thermal, optical, tribological, magnetic, moisture sensitivity anddielectrical properties. The list of monomers, monomer mixtures andmonomer/crosslinkers thus would embrace all such materials known or newmonomers to be synthesized.

Alternatively, as already noted, cavity 116 can be in any shape capableof being used as a mold. The advantage to differential polymerization isthat one obtains precision parts that are stress free and flawless.

Referring now to FIG. 5 a device for solidification of a liquefiedpolymer mixture by the application of a solidifier is shown. As in theembodiment just discussed a mold structure 10' is utilized in thisembodiment. Mold structure 10' has a first part 12, and a second part14' with second part 14' being slidably removable from the first part12'. In this embodiment a liquefied polymer mixture, that consists of asolid polymer and perhaps other fillers that have been dissolved in asolvent is contained in reservoir 20, which is in communication withcavity 16'. Cavity 16' is filled with the liquefied polymer when secondpart 14' fully closes cavity 16'. In the mold structure shown in FIG. 5a flat plate like structure or sheet will be the result of the castingprocess. Should drying or devolatilization take place simultaneouslyover the entire flat plate or sheet, the sheet will warp or crack asshrinkage occurs due to solvent evaporation. Equivalently, when anonsolvent extractant is used to leach out the solvent as incoagulation, a certain degree of shrinkage will result depending on therelative rates of solvent leaving the mixture and nonsolvent enteringthe mixture. If, however, second part 14' is withdrawn slowly from themold body 10' so that devolatilization and/or coagulation can occur in adifferential manner such as described above with the polymerizationprocess, then the dissolved polymer feed contained in reservoir 20 canflow into the mold cavity 16' to fill the spaces that occur because ofthe shrinkage. While shrinkage may not be as great in thedevolatilization or coagulation of a liquefied polymer as inpolymerization, it is sufficiently significant that cracking will occurin the finished sheet. Should it be necessary to evacuate the spaceabove mold cavity 16' or to use some atmosphere other than ambient airor to use a liquid nonsolvent or nonsolvent vapor, a vacuum pump orsource of solidifier 22' may be affixed to a chamber that surrounds themold body 10'.

Referring now to FIG. 7, a schematic is shown for a device used fordevolatilization of a liquid polymer to form a curved part. Inparticular mold body 10" is formed with a first part 12" and second part14" (See FIG. 8) forming a curved cavity 16". In this particularembodiment second part 14" swings from an axle or pivot point 52 todifferentially expose mold cavity 16" to an evaporating atmosphere. Thereservoir 20" is filled with a liquefied polymer and communicatesthrough a conduit 54 with a gate 56 to ensure that mold cavity 16" iscontinuously filled with liquefied polymer. The liquefied polymer may besubsequently devolatilized by the withdrawal of second part 14" in asequential or differential manner as described above.

It should be understood that application of this invention to any shapeor mold to form precision parts by either in situ polymerization ordevolatilization or coagulation of a pre-solidified polymer feed islimited only to the extent of the capacity of the mold maker. Forexample, it is specifically addressed toward but not restricted toprecision parts such as lenses and compact discs, and lightweightstructural parts that require precision molding and must be stress free.

OPERATION OF THE EMBODIMENTS

Operation of the aforedescribed invention should be clear to thoseskilled in the art, however, a brief review is offered forconsideration.

Referring to FIG. 1, the mold 10 is clamped together in a conventionalmanner with reservoir 20 in the position shown. Reservoir 20 is filledwith the reacting mixture in this case a monomer, a mixture of monomersor a monomer/crosslinker mixture loaded with an initiator and/or othercatalysts, such that the material will easily flow into cavity 16. It isimportant to ensure that cavity 16 is fully filled with the reactingmixture before polymerization is attempted. Accordingly it may beappropriate to provide a vent 21 to the mold cavity 16. In the event avent is employed, it should be closed and plugged before polymerizationtakes place. Closing the vent will assist in drawing additional reactionmixture into cavity 16 during polymerization rather than permitting airto enter the mold.

Once mold cavity 16 is filled, the source of energy 22 may be activatedand focusing gate 24 moved relative to mold body 10 thereby imposingeither heat or light, as appropriate, to the mold body in a differentialmanner. Should heat be the source of energy, then it may be appropriateto activate cooling passages 34 at the upper end of the mold body toensure that heat conduction through the mold body will not initiatepolymerization in the upper portion of the mold before the focusing gate24 traverses the entire face of the mold.

Once focusing gate 24 has completed its passage and polymerization iscomplete in the mold body 10, then the mold structure can be taken apartand the molded precision part removed.

Operation of the embodiment shown in FIG. 3 follows the same pattern asthat described above and will not further be described herewith.

In the devolatilization or coagulation differential casting processshown in FIGS. 5, 6, 7, and 8, the liquefied polymer mixture containedin reservoir 20' or 20" is allowed to flow into the mold space 16' or16" as appropriate and completely fill the mold cavity. Once the moldcavity is completely filled, then withdrawal of the second part 14' toexpose the liquefied polymer differentially to the solidifyingatmosphere or nonsolvent may be accomplished. The rate of removal of thesecond part 14' is dependent upon the liquefied polymer to besolidified. This of course will vary with the different materialsutilized and in part may be dependent upon the thickness of thematerial. In FIGS. 7 and 8, the same procedure is followed except thatthe second part 14" is swung away sequentially to form a curved shape asindicated.

While this invention has been described in relation to certainembodiments, it is not to be so limited, rather, it is limited only tothe extent of the appended claims.

What is claimed is:
 1. A method of forming an article of precisedimensions by solidification in a precisely dimensioned mold of aliquefied material comprising at least one monomer, prepolymer orpolymer, that, upon exposure to a solidifying source of electromagneticradiation, shrinks; the method comprising the steps of:a. providing amold body having a first part and a second part, said first and secondparts defining an internal cavity therebetween, the cavity correspondingto the precise dimensions of the finished article and having a closedend and an open end, at least one of said first or the second partsformed to permit exposure of the liquefied material to theelectromagnetic source in the internal cavity in a differential andsequential manner; b. providing the electromagnetic source forimposition through a focusing gate upon the surface of the liquefiedmaterial to the electromagnetic source in the internal cavity in adifferential and sequential manner; c. providing a source of theliquefied material at said open end of said mold body; d. filling theinternal cavity with the liquefied material; e. differentially exposingsaid liquefied material to said electromagnetic source through thefocusing gate by moving the source relative to the mold body starting atsaid closed end and proceeding to the open end while continuouslysupplying liquefied material to said open end so that polymerizationtakes place at a controlled rate from the closed end to the open end;and f. removing the resultant solidified article from the cavity.
 2. Themethod of claim 1 wherein the solidifying electromagnetic source is aheat source.
 3. A method of polymerization of a reaction mixture withina precisely dimensioned mold to form an article by imposition of a formof energy on the reaction mixture, the method comprising the followingsteps:a. providing a mold body of at least two parts and having at leasta portion thereof permeable to the form of energy, the mold defining aninternal cavity having a closed first end and a second end, said cavityconforming to the outer dimensions of the article, and the mold bodyfurther defining a gate at said second end of said internal cavity; b.providing a source of the form of energy for permeating a definedlocalized area of the permeable portion of the mold body; c. providing asource of reaction mixture capable of being polymerized by theimposition of a form of energy; d. filling the internal cavity with thereaction mixture; e. differentially exposing said reaction mixture, bylocalized portions through a focusing gate, to the one form of energystarting at the first end of said internal cavity and proceeding on atimed sequence basis to the second, gated end whereby the reactionmixture is polymerized from the first end to the second, gated end at acontrolled rate; and f. removing the resultant polymerized article fromthe cavity.
 4. The method of claim 3 further including the step ofproviding a source of heat to act as the one form of energy.
 5. Themethod of claim 3 further including the step of providing a source ofelectromagnetic radiation as the one form of energy.
 6. The method ofclaim 3 wherein the step of exposing differentially the reaction mixtureto the one form of energy includes the step of moving the mold and thesource of energy relative to one another.
 7. The method of claim 3wherein the step of providing a source of energy includes a step ofproviding a source energy so that the energy radiates from the sourcegenerally in a plane that is perpendicular to the relative movement ofthe mold and the source of energy.